The present invention relates to an apparatus for irradiating a specimen with a beam of charged particles such as ions or electrons.
The following explanation of a charged particle beam apparatus will be made taking a focused ion beam apparatus by way of example. A focused ion beam apparatus includes a high-intensity ion source such as a liquid metal ion source and an ion optical system for focusing and deflecting ions to irradiate any given position on a specimen with a beam of ions. The ion source and the ion optical system are provided in a vacuum vessel. The conventional focused ion beam apparatus is discussed by, for example, J. Vac. Sci. Technol. B4 (1), 1986, pp. 189-193.
The above reference discloses a focused ion beam apparatus which has a basic structure as shown in FIG. 1. Electrostatic lenses 3a and 3b form an image of an ion source 1 on a specimen 6. The diameter of an ion beam 2 on the specimen is mainly determined by the aberration of the electrostatic lenses and depends upon the spread angle of an ion beam on the specimen. Accordingly, a beam current and the beam diameter can be changed by selecting a proper aperture of a beam current limiting aperture plate 5, including a plurality of apertures 10, the sizes of which are different. Namely, a proper one of the apertures 10 is selected by moving the aperture plate 5 in a left/right direction in FIG. 1 by means of a known fine adjustment or movement mechanism. However, in thus selecting the aperture, usually the center of the aperture deviates from a beam axis due to a minute error associated with the fine adjustment mechanism of the aperture plate 5, thereby increasing the aberration so that a beam distribution on the specimen asymmetrically spreads, as shown in FIG. 2. Therefore, it is necessary to carry out the complicated fine adjustment of the aperture position for several ten seconds to several ten minutes every time there is an aperture change, while judging the beam distribution by virtue of an image of secondary electrons emitted from the specimen irradiated with the ion beam. In FIG. 1, reference numeral 4 designates an electrostatic deflector for deflecting the ion beam 2, numeral 8 a secondary-electron detector for detecting secondary electrons emitted from the specimen 6, and numeral 7 a specimen stage for holding the specimen 6 thereon and providing a small movement to the specimen 6.
In the above prior art, though it is possible to change the beam current and the beam diameter of the ion beam with which the specimen is irradiated, the execution of such change in a short time is difficult since the complicated aperture positioning is required.